The present invention provides new nanocomposites comprising a graphitic matrix in which nanosized iron fluoride or iron oxide particles are embedded. The invention further comprises a one-step method for preparing said composites and their use as electrode material.
Lithium ion batteries are key energy storage devices that power today's consumer electronics. However, their energy density still fall short for transportation and large scale power storage applications. One way to increase the energy density of battery is to use high energy density electrode materials. The present commercial Li-ion batteries use LiCoO2 or LiFePO4 based insertion positive electrode materials. While LiCoO2 is a layered compound with a specific capacity of 150 mAh/g, LiFePO4 is a framework compound whose capacity is 170 mAh/g. Even though both compounds show excellent reversibility with lithium, the specific capacity is limited by single electron redox reaction per molecule or even less.
A valid approach to increase the energy density of electrode material is to utilize all possible redox states of metal ion. The best candidates for this purpose are metal fluorides as they reversibly react with lithium at relatively high voltage (H. Li, J. Richter and J. Maier., Adv. Mater. (2003), 15, 736-739). However, to their disadvantage metal fluorides are electrical insulators. Further, when micron sized metal fluoride particles are used the capacity fades rapidly with cycling.
Among various metal fluorides, iron fluorides are important class due to their low cost and low toxicity. In this context, FeF2 is an interesting cathode material which has a thermodynamic reduction potential of 2.66 V versus lithium and has a theoretical specific capacity of 571 mAh/g to a gravimetric energy density of 1519 Wh/kg. However, FeF2 is an electrical insulator and needs to stay in intimate contact with electronic conductors in order to become electrochemically active.
To address these problems carbon-metal fluoride nanocomposites (CMFNCs) are proposed in U.S. 2004/0062994. These composites are prepared using mechanical high-energy milling of FeF2, FeF3, NH4FeF4, NiF, or CoF and activated carbon, carbon black, or expanded graphite. The total carbon content in the nanocomposites comprises about 5% to 50% by weight. Similar composites and a method for their preparation are described by Badway et al. (F. Badway, N. Pereira, F. Cosandey and G. G. Amatucci J. Electrochem. Soc., (2003), 150 (9), 1209-1218). However, the preparation of graphitic carbon-metal fluoride nanocomposites by simple milling leads to less stable interfaces between carbon and the respective metal compound. Hence, carbon may detach from the active material which is expanding and shrinking during charge-discharge cycles, so that more and more volume elements of the composite become inactive.
Plitz et al. presented a method for synthesising Carbon-Metal Fluoride Nanocomposites (CMFNCs) starting from insulative carbon fluoride (CF) as oxidizing agent and FeF2, NiF2, or CoF2 precursors (I. Plitz, F. Badway, J. Al-Sharab, A. DuPasquier, F. Cosandey, G. G. Amatucci “Structure and Electrochemistry of Carbon-Metal Fluoride Nanocomposites Fabricated by Solid-State Redox Conversion Reaction” (2005) Journal of the Electrochemical Society, 152(2) 307-315). Unfortunately, ball milling destroys any complex microstructure and the small nanocrystals can agglomerate much more easily. This state of the art system contains relatively large FeF3 nanoparticles in a range of about 20 nm. Moreover, the ball milling of active material with conductive carbon leads to nanocomposite structures where carbon flakes are weakly attached to the nanoparticles or structures where the particles are sitting loosely on top of the carbon/graphene surface. In all of these materials degradation occurs during charging and draining because the interface is not stable enough to deal with the associated volume expansion and shrinkage of the active electrode material, in particular when conversion materials are used.
A further disadvantage of the state of the art is the lack of an economic one-step synthesis which is easy to perform. The cited state of the art does not disclose iron nanoparticles or nanocomposites clamped into a highly conducting graphitic matrix.